This invention relates to transient voltage surge suppression (TVSS) and, more particularly, to a ground to neutral fault sensing apparatus for use in a transient voltage surge suppressor.
Transient voltage surge suppressors are used to protect electrical components and systems from electrical transients and surges. They are used with computers and similar electronic equipment to prevent system upset as well as catastrophic failures caused by lightning, for example. Typically, TVSS components are connected in parallel to power distribution lines to protect the electrical loads represented by this equipment from damage. A description of a modular TVSS for this type of protection may be found in U.S. patent application 08/109,815 filed Aug. 19, 1993 and assigned to the same assignee as the present application.
One critical element of any TVSS unit is monitoring the integrity of the surge protective components. TVSS modules such as those disclosed in the co-pending patent application 08/109,815 include components such as metal oxide varistors (MOV's) which operate in a non-linear manner. That is, they switch from an OFF to an ON mode of operation when their threshold voltage level is exceeded. When ON, their resistance goes from a very high value to a very low value. In the circuit configuration in which they are connected, they then serve to effectively divert surge currents away from the protected load and clamp transient voltages.
MOV's used in TVSS modules are limited as to the amount of current they can divert. For example, a disk type MOV having a diameter of 20 mm is capable of diverting a 20 microsecond pulse having a peak amplitude of up to 6,500 amps. If the peak current exceeds this value, the MOV may fail. When a MOV fails, its typical failure mode is to fail short. Even after its failure, if it has failed short, it will continue to draw current due to the line voltage, heat up, and then explode catastrophically. This results in an open circuit. To prevent this from occurring, an appropriately sized fuse is connected in series with the MOV. Now, if the MOV shorts, the current drawn by the MOV is sufficient to clear the fuse. This results in an open circuit but not one produced by a catastrophic MOV failure which could cause other damage. Also, if the surge current is significantly higher than the maximum allowable for the MOV, the fuse may clear before the MOV fails short. For surge protection between neutral and ground there is no follow current.
When a TVSS is installed so as to provide phase-to-neutral, phase-to-ground, or phase-to-phase protection, a phase voltage between a fuse link and the MOV can be monitored. If the MOV fails, subsequent clearing of its associated fuse results in a reduced voltage. This voltage reduction is a clear indication of MOV failure and the need for appropriate action. However, for a TVSS installed between ground and neutral, a phase voltage which indicates MOV failure cannot be monitored. What is needed therefore is a fault sensing circuit which provides a clear and unequivocal indication of when a surge or transient has caused an MOV failure between neutral and ground.